Properly paint QListView dragged item in icon mode

Currently, when dragging a QListView item in icon mode, only the
item is moved and once out of the view port, the visual feedback is
lost. This patch updates the QDrag pixmap to have a persistent view of
what is moved

Based on Qt 5 c3bf3bd8b74187b44ec91582e1cf2be546a73349

Task-number: QTBUG-1180

[ChangeLog][QtGui][QTBUG-1180] Dragging an item outside the QListView in icon mode doesn't lose the icon.

Change-Id: I9d7fee4c31a4d3d5467510f16fd573635eb6d6f0
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>